1. Field of the Invention
The present invention relates generally to an apparatus for screening magnetic and/or electromagnetic fields, and specifically to such apparatus that are composed of a plurality of panels, constructed of layers of soft-magnetic material having an initial permeability of more than 3,000, and optionally of additional metallic or non-metallic layers.
2. Description of the Related Art
German OS 24 46 986 describes an apparatus for screening electromagnetic material in a walk-in compartment. The compartment is formed with pre-fabricated, completely annealed sheets of a soft-magnetic nickel-iron alloy that are glued onto a wood cell lined with plywood. One disadvantage of this structure is that stresses in the highly sensitive magnetic material occur due to the gluing of the screening plates onto plywood. Such stresses lower the permeability values of the magnetic material.
OS 24 46 986 proposes to mitigate these stress problems by providing a supporting skeleton in which all load-bearing members are screwed to one another. The inside of the skeleton is then lined with deep-drawn sheets of carrier material in order to obtain optimally planar wall surfaces. The actual screening plates disclosed are preferably composed of a nickel-iron alloy that contains 70 through 82 weight percent nickel, 0 through 11 weight percent copper, 0 through 6 weight percent molybdenum, 0 through 6 weight percent chromium, the remainder iron, whereby the iron content amounts to at least 9 weight percent. The prefabricated screening plates have exemplary dimensions of 2500 mm .times.380 mm.times.2 mm, and are completely annealed in an annealing furnace. The walls, ceiling, and floor of the space to be screened are then covered cross-wise with two layers of screening plates. The two sub-layers of magnetic screening material are then secured to the deep-drawn sheets with hollow blind riveting, in order to gently join the highly permeable screening plates to one another.
A publication of K. J. Best and J. Bork, in ETZ, Vol. 110, No. 16, pages 814ff (1989), describes screening compartments that are also based on the principle of sub-layer systems of highly permeable magnetic material laid cross-wise to one another. Different materials are employed in each shell of the three-shell standard compartment described therein. The respective individual shells are composed of highly permeable magnetic material and of highly electrically conductive material (i.e., aluminum). The magnetic walls are constructed of large panels, with the afore-mentioned strips of magnetic material overlapping one another cross-wise on the panel surfaces. The entire structure is in turn held by a framework of lightweight structural members. This publication emphasizes that the highly permeable magnetic material must be protected against mechanical overstressing.
The Best publication also describes a "special project": a screening compartment that has a seven-shell structure. In this compartment, the six outer shells are composed of mu metal and the inner shell is composed of copper. The compartment does not use a carrier system, since the shells are supported against one another by non-magnetic intermediate layers. The need for optimally great rigidity, particularly dynamic rigidity, is of special significance in multi-panel screening compartments. Greater rigidity decreases the microphone effects due to panel edge oscillations. The vibration-free suspension of measuring instruments (for example, of a SQUID with a mount at the compartment ceiling) is also of significance. The increasing use of highly sensitive measuring systems serves to emphasize the need for more rigid compartment structures.
Attempts were made in the screening compartment described in the Best publication to take the known stress-sensitivity of soft-magnetic material into consideration, with the conclusion that it was optimal to "separate" the screening material from the supporting structure. The publication refers to a "floating" fastening, i.e. the screening material is only loosely secured to the structure (by hollow blind riveting, as described in German Published Application 24 46 986 upon which the screening compartments of the Best publication are based). The "floating" fastening ensures that the screening material does not bear any auxiliary loads or moments other than the dead weight of the screening material itself. The support load-bearing is performed by discrete carrier elements, or by non-magnetic intermediate layers.